Experimental Study Of Premixed N-heptane/air Catalytic Combustion Characteristics In Micro-cylindrical Tube

With the rapid development of micro propeller,micro sensor technology,micro electronic mechanics and micro machining technology,power source from MW to W is in growing demand.Traditional rechargeable power supply limited by energy density and recharge time,therefore is not the optimal choice.Combustion-based micro system has the advantage of small volume,high energy density,long lasting time and convenient refuel,therefore they have become the subject of many investigations.With the decrease of combustor size,surface to volume ratio increased rapidly and combustion stability and combustion efficiency would change a lot.Dimentional effect on catalytic combustion is one of the most key issues of micro scale combustion system.Focused on the scientific problem,n-heptane catalytic combustion and dimentional effect have been ivvestigated to get the varation of inlet velocity and equivalence ratio（Φ）in catalytic combustion characteristic and dimentional effect.Catalytic combustion on Pt/Ce_0.8Zr_0.2O₂ and Ce_0.8Zr_0.2O₂ were performed to investigate the effect of Ce_0.8Zr_0.2O₂ on n-heptane catalytic combustion.Quart tube with inner diameter of 4mm was chosed as the combustor.Experiment results show that,with the help of Pt catalyst,stable catalytic combustion of n-heptane can be achieved when equivalence ratio was less than 2.Without Pt catalyst,stable combustion was hardly to sustain when equivalence was less than 2.These results illustrate that Pt catalyst is useful to enlarge stable combustion of n-heptane.Tube wall temperature and combustion efficiency of n-heptane catalytic combustion with Pt catalyst were higher than that of working condictions without Pt catalyst,which indicate the fact that Pt catalyst is the decision factor of the whole reaction.Ce_0.8Zr_0.2O₂ supported Pt catalyst was arranged as Packed-bed in micro-cylindrical quart tube to investigate premixed n-heptane/air catalytic combustion characteristics,including catalytic activity,combustor wall temperature,maximum tube wall temperature,conversion rate and surface heat loss.Catalytic activity experiment showed that ignition temperature of n-heptane in Φ = 0.5 was 452 K,and catalytic combustion can be achieved under 500 K.In self-sustaining experiment,maximum wall temperature shifts downstream and high temperature zone expands.Maximum tube wall temperature of Φ = 0.5 and 1 experienced sharp drop in Re = 125.When Re is fixed,working condition of Φ = 1 acquires higher maximum wall temperature and conversion rate,lower surface heat loss than that of Φ = 0.5,which means working condition of Φ = 1 is more suitable in micro-combustors.n-Heptane combustion in cylinder tubes with diameters of 2,3,and 4 mm was investigated experimentally with the use of packed catalyst beds of Pt/Ce_0.8Zr_0.2O₂ to study the mechanism of self-sustaining combustion and the dimensional effects of the inner diameter of a tube combustor.The region of self-sustaining stable combustion,the tube wall temperature,and the combustion efficiency were examined,n-heptane can maintain self-sustaining stable combustion in the three tubes even if the wall temperature was below 430 K.As the equivalence ratio（Φ）increased,the stability region of the Reynolds number（Re）was expanded at the expense of fuel wastage.The lower stability limit of Re decreased as the tube diameter increased,and the value could reach as lows as 12 in the tube with a diameter of 4 mm.The equivalence ratio was the dominant factor of wall tube temperature at the lower boundary,whereas Re was significant at the upper boundary.An optimal Re allowed for the achievement of the highest wall temperature,and the optimal Re was high with a small diameter.The tube with a large diameter presented both high combustion efficiency and high heat release rate.Combustion efficiency was improved by 40%when the diameter of the tube increased from 2 mm to 4 mm,and over 80%combustion efficiency was achieved in the d = 4 mm tube at Φ = 2.